1. Technical Field
The present invention relates to a position measuring arrangement including a code having a sequence of code elements arranged one behind the other in the measuring direction, in which several sequential code elements each constitute a code word containing absolute position information. The arrangement further includes a scanning arrangement, having a detector unit with several detector elements for scanning the code elements, each of which constitutes a code word, and for forming at least one scanning signal within respectively one of the code elements. An evaluating unit with assessment arrangements, in which the at least one scanning signal of a code element is supplied to respectively one of the assessment arrangements, and by which information for the corresponding code word can be formed by at least one reference value.
2. Background Information
Absolute position measuring arrangements are increasingly employed in many fields, in which absolute position information is derived from a code track with code elements, which are arranged one behind the other in the measuring direction. In this case, the code elements are provided in a pseudo-random distribution, so that a defined number of code elements following each other forms a respective bit pattern. In the course of the displacement of the scanning arrangement by a single code element with respect to the code track, a fresh bit pattern is already formed, and a sequence of different bit patterns is available over the entire measuring range to be detected in an absolute manner.
Such a sequential code is called a chain code or a pseudo-random code (PRC).
A position measuring arrangement with such a code is described in GB 2 128 444 A. The information regarding each code word of this code is determined by comparing the scanning signal of the code word with a fixed, preset reference value. A logical “0” is formed, if the scanning signal lies below the preset reference value, and a logical “1” is formed, if the scanning signal lies above the preset reference value.
A position measuring arrangement with Manchester coding is described in DE 102 44 235 A1. This position measuring arrangement has a code including a sequence of code elements arranged one behind the other in the measuring direction, in which each code element respectively includes two partial areas which are complementary to each other and are arranged following each other in the measuring direction. This code is scanned by a scanning arrangement by several detector elements. In a comparison arrangement a comparison result is formed from the respective scanning signals from the partial areas of a code element, and a check is made by a checking arrangement whether the result of the comparison lies above or below a fixedly preset reference value, and depending on this, a bit value “0” or “1” is derived as information from the respective code element. Additionally, a check is made whether the result of the comparison lies within or outside of a range predetermined by a further reference value. Depending on the result from this check, further information regarding the code word is derived. If the result of the check lies within the predetermined range, an error signal is generated for the respective code element, which states that the bit value derived from this code element is unreliable.
In connection with this, the reference value for forming the bit value is fixedly preset. The range for checking the reliability of the bit value is also fixedly preset once.
This has the disadvantage that, in case of a reduction of the amplitudes of the analog scanning signal in the course of operation, error signals are increasingly generated.
A position measuring arrangement with a serial, or respectively sequential code, is also described in JP 1-318920 A. The code includes two code tracks arranged parallel with respect to each other. One of these code tracks has a sequence of code words, which are arranged in the measuring direction and form bit patterns, which can be unequivocally differentiated from each other, over the entire measuring range. The code elements of the second code track arranged next to it are designed inversely to the code elements of the first code track, and a bit “0” or “1” of a code word is respectively determined by forming the difference between the code elements, which are located next to each other and are formed inversely with respect to each other.
A step for adapting the comparison threshold for the scanning signals of the serial code to the conditions of the surroundings is disclosed in EP 0 841 538 B1. A detector is provided for this, which scans a location outside of the sequential code and provides a signal, by which the comparison threshold can be adapted.
It is disadvantageous in this case that the states detected by this additional detector do not correspond to the states at the location of the code, so that erroneous interpretations of the code elements can occur.